There has conventionally been a need to appropriately perform color correction (color information correction) on image signals in order to improve the image signal visibility, for example.
For example, one issue is that, when viewing the display screen of a portable telephone, for example, outdoors during the daytime or on the train, the direct sunlight, etc., may cause the display screen to become saturated white, and this makes it difficult to view the information that is displayed on the display screen.
Generally speaking, there is a substantial demand for improvements to this situation. To meet this demand, many undertakings for improving the liquid crystal device characteristics of the display screen are under investigation. Of such undertakings, one approach that is generally known is the method of changing the liquid crystal transmissivity of the display screen according to the ambient light level of the surroundings. Devices that use this method are known to improve the ratio of the white pixels with the highest luminance to the black pixels with the lowest luminance (the contrast ratio) to about 20:1 to 15:1. However, the aim of such devices is to improve the visibility with regard to the luminance, and they do nothing to improve the loss of color information, such as color difference, hue, or color saturation, due to strong ambient light such as sunlight during the day. Further, it has been pointed out that the white to black contrast ratio falls to about 3:1 to 2:1 during the afternoon, and thus even using a device that is capable of contrast ratios of about 20:1 to 15:1 often may not produce a sufficient improvement in the visibility in terms of the luminance.
Thus, to improve the visibility of a display screen in strong ambient light conditions, it is necessary to correct the color information as well as the luminance.
Improvements in display device performance (i.e. improvements in resolution, an increase in the number of expressible colors, and improvements in the performance of color correction centered on memory colors) have increased user awareness of the color information, and in cases where color information is to be corrected, color correction that is more natural and compatible with human visual characteristics is desirable.
For example, one image processing method takes into account the color contrast characteristics of human vision. With this image processing method, the foreground and the background of an image are extracted based on the hue histogram and the pixel area of the image, and the hue of the background with respect to the foreground is corrected uniformly (for example, see Patent Document 1). Here, the goal is to keep a low level of color in the background with respect to the foreground, in consideration of human color contrast characteristics.
An image processing device that achieves this image processing method is described using FIG. 30. FIG. 30 is a block diagram of a color correction determination portion 2005 that performs color correction determination in the image processing device. As shown in FIG. 30, the color correction determination portion 2005 is furnished with an image memory 2000, a solid image determination portion 2001, a histogram creation portion 2002, a foreground/background discrimination portion 2003, and a color number discrimination portion 2004. The image memory 2000 holds the images. The solid image determination portion 2001 determines whether an image is a solid image based on whether or not a number of hue regions that is equal to or greater than a certain threshold value are continuous over at least a predetermined number of pixels in a predetermined hue histogram. The histogram creation portion 2002 creates histograms of the hue, color saturation, and color value of the solid image. The foreground/background discrimination portion 2003 determines the foreground, background, and others based on the histograms that have been created. The color number discrimination portion 2004 determines whether or not the image is a full-color image.
FIG. 31 shows a flowchart of the image processing method achieved by this image processing device, and FIG. 32 shows a flowchart of hue correction.
The image processing method shown in FIG. 31 is described. In the image processing method, first the color number is ascertained (S100), and then whether or not the color number exceeds a threshold value Th1 that indicates “full color” is determined (S101), and if the image is full color, then the procedure advances to S102, and if it is not full color, then the procedure is ended.
In S102, the solid image portions are determined, and in S103, histograms H(h), H(v), and H(s) of the hue h, the color value v, and the color saturation s of the solid image portions are created. Next it is determined whether the values of the histograms exceed a threshold value Th2 (S104). If the result of this determination is yes, then this determination is continued until it has been performed for all the values (S105). It should be noted that FIG. 31 shows only the hue h and the histogram H(h), but this determination is also performed for H(v) and H(s) as well. When the determination has been performed for all values (S105), it is determined whether there is more than one H(h), etc., that exceeds the threshold value (S106).
If the result of this determination is yes, then it is determined whether those belong to the foreground, the background, or to another region that is neither the foreground nor the background (S107). If the result of this determination is yes, then the color correction process S108 shown in FIG. 32 is executed. This process is not executed if the result of any one of the determinations in S101, S104, S106, or S107 is no. Due to this processing, if the area occupied by a foreground color and a background color is equal to or greater than a predetermined size, then the appropriate color correction is performed, whereas color correction is not performed if this is smaller than the predetermined size.
The color correction process shown in FIG. 32 is described.
First, the hue (ha) of the background color is extracted (S200) and the correction level Aha of ha in the color difference axis direction is extracted (S201). Next, in S202, the hue (hf) of the foreground color is extracted, and hf+Aha is set as the new hue hf of the foreground color (S203).
By performing this process, the goal of the above conventional technology is to nullify the effects of the foreground portion taking on color in the direction that the background portion is corrected due to hue contrast phenomena, and thereby faithfully make the color reproduction.
With this conventional technology, it is necessary to accurately extract the solid image regions that have a predetermined pixel size and that are included in a particular hue region, and it is also necessary to accurately determine whether or not the solid image regions are in the foreground or in the background.
[Patent Document 1] JP 2001-292333A